Methods and circuits for adaptive equalization

ABSTRACT

An integrated circuit equalizes a data signal expressed as a series of symbols. The symbols form data patterns with different frequency components. By considering these patterns, the integrated circuit can experiment with equalization settings specific to a subset of the frequency components, thereby finding an equalization control setting that optimizes equalization. Optimization can be accomplished by setting the equalizer to maximize symbol amplitude.

FIELD OF THE INVENTION

The present invention relates generally to the field of communications,and more particularly to high speed electronic signaling within andbetween integrated circuit devices.

BACKGROUND

Serial communication links that employ channels that exhibit low passfilter effects often use transmit pre-emphasis, receiver equalization,or a combination of the two to overcome the loss of high-frequencysignal components. Adaptive transmit pre-emphasis or receiveequalization may be used for marginal links or links whose transfercharacteristic change over time. In either case, the received signalquality may be measured at the receiver. Adaptive transmit pre-emphasisschemes may therefore use some form of back-channel communication torelay indicia of signal quality back to the transmitter. Unfortunately,the need for a backchannel renders the design and implementation ofadaptive pre-emphasis challenging and complex. Also important, someintegrated circuits that receive data via a serial link may not includea compatible backchannel receiver with which to communicate. Thetransmit and receive circuitry may be parts of integrated circuits fromdifferent vendors, for example, in which case the two vendors would haveto agree in advance upon a backchannel communication scheme and designtheir circuitry accordingly. Such collaboration may be impractical.

Adaptive receive equalization does not require backchannelcommunication, and thus avoids many of the problems inherent in adaptivetransmit pre-emphasis. Optimum pre-emphasis and equalization settingsare data specific, however, because different data patterns havedifferent spectral content, and thus are affected differently bylow-pass characteristics of the channel. As a first-order approximation,the higher the frequency, the greater the attenuation. Transmitters“know” the transmitted data pattern in advance, and thus can tailor thetransmit pre-emphasis to the data; in contrast, receivers do not knowthe received data pattern in advance, so adaptive equalization thataddresses changes to the incoming data is much more difficult.

Some adaptive receive equalization schemes measure the power density ofreceived signals at two frequencies and adjust the receive equalizer tomaintain some desired ratio of the two power densities. Unfortunately,such schemes may not provide appropriate levels of equalization forfrequencies other than those monitored. Furthermore, noise at amonitored frequency contributes to the measured power density, andconsequently results in erroneous equalizer settings. There is thereforea need for receive equalization systems and methods that are moreresponsive to received data patterns and less sensitive to noise.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 depicts a communication system 100 in accordance with oneembodiment.

FIG. 2 depicts a receiver in accordance with an embodiment.

FIG. 3 depicts a flowchart illustrating a convergence algorithm 300 thatmay be used by adaptive control logic 145 and amplitude detector 140 ofFIG. 1 or 2 to select an equalization setting for equalizer 125, inaccordance with some embodiments.

FIG. 4 is a flowchart illustrating a tracking algorithm 400, which maybe used by adaptive control logic 145 of FIG. 1 or 2 in accordance withsome embodiments.

FIG. 5 schematically depicts an equalizer that may be used to implementequalizer 125 in accordance with one embodiment.

FIG. 6 schematically depicts a bias-voltage generator for use withequalizer 125 of FIG. 5.

FIG. 7 schematically depicts a DAC and sampler that may be used toimplement DAC 220 and sampler 215 of FIG. 2 in accordance with oneembodiment.

FIG. 8 details an embodiment of clock reduction circuitry that may beused to implement the clock reduction circuitry 200 of FIG. 2, whichreduces the frequency of data clock Dclk by a factor of e.g. four andcreates sample clock Sclk edge aligned with data clock Dclk.

FIG. 9 depicts data filter that may be used to implement the data filter150 of FIG. 1 in accordance with one embodiment.

DETAILED DESCRIPTION

FIG. 1 depicts a communication system 100 in accordance with oneembodiment. System 100 includes a transmitter 105 that transmits adifferential data signal Vin (Vin_p/Vin_n) to a receiver 110 via adifferential channel 115. A conventional transmitter may be employed astransmitter 105, so a detailed treatment is omitted here for brevity.Transmitter 105 optionally includes transmit pre-emphasis circuitry todynamically adjust the data signal Vin to reduce signal distortioncaused by the effects of channel 115. Such transmit pre-emphasiscircuitry may include, for example, a multi-tap transmit amplifier 120adapted to cause the voltage amplitudes of the data symbols of signalVin to be selectively increased or decreased based on the data values ofpre and/or post cursor data symbols.

Communication system 100 also includes a receiver 110 that receives datasignal Vin. Receiver 110 includes an equalizer 125 that equalizes datasignal Vin to produce an equalized signal Veq. Equalizer 125 adjusts themagnitude (e.g., voltage and/or current) of at least some data symbolsin data signal Vin. In some embodiments, equalizer 125 selectivelyadjusts the voltage amplitude of at least some of the data symbols indata signal Vin. In some embodiments, equalizer 125 selectively adjuststhe current used to express at least some of the data symbols in datasignal Vin. In one embodiment, equalizer 125 receives signal Vin, via adifferential input port, and amplifies signal Vin using a range ofamplification factors, with higher frequency components of Vin beingtreated to higher amplification factors. If channel 115 exhibits a lowpass filter effect, then such an equalizer may be used to, for example,compensate for the low-pass nature of channel 115. In that case, thedegree to which equalizer 125 amplifies higher frequency signalsrelative to lower frequency signals can be adjusted via an equalizerinput port Eq. A conventional sampler 130 samples the equalized signalVeq in synchronization with a data clock Dclk to produce a first sampleddata signal Din. Data clock Dclk is, in this example, recovered from theinput data using a conventional clock-and-data recovery circuit (CDR)135. A sampler suitable for use as sampler 130 is described in“0.622-8.0 Gbps 150 mW Serial IO Macrocell with Fully FlexiblePreemphasis and Equalization,” by Ramin Farjad-Rad, et al. (2003Symposium on VLSI Circuits Digest of Technical Papers), which isincorporated herein by reference. Other suitable receive samplers mightalso be used.

An amplitude detector 140 periodically samples, in synchronization withclock signal Dclk, the symbol amplitude Sa of equalized input signalVeq. Some adaptive control logic 145 then calculates the appropriateequalization setting based upon measured symbol amplitudes and adjustsequalizer 125 accordingly. An equalization setting may thus be selectedto maximize the amplitude of sampled data at the appropriate sampleinstant. Receiver 110 additionally includes a data filter 150 thatselectively enables amplitude detector 140. Data filter 150 causesamplitude detector 140 to measure and record the amplitude of a subsetof possible data patterns, such as those associated with higherfrequencies.

FIG. 2 depicts portions of receiver 110 of FIG. 1, in accordance withone embodiment, like-labeled elements being the same or similar. FIG. 2additionally depicts clock reduction circuitry 200 that reduces thefrequency of data clock Dclk by e.g. a factor of four to ease theimplementation of the adaptive control circuits and logic. For example,in an embodiment in which the frequency of data clock Dclk is 3.125 GHz,clock reduction circuitry 200 divides data clock Dclk by four to producea 781 MHz sample clock Sclk. Using this lower sample clock frequency,the circuitry of amplitude detector 140 and adaptive control logic 145can be synthesized using a standard cell library for significantlyreduced design time and improved efficiency. Clock reduction circuitry200 includes a clock divider 205 that divides the frequency of the dataclock by a factor K (where in the embodiment depicted in FIG. 2, K=4) toproduce an intermediate clock signal Pclk and an edge aligner 210 thataligns intermediate clock Pclk with data clock Dclk to produce a sampleclock Sclk.

Amplitude detector 140 includes, in this embodiment, a sampler 215, adigital-to-analog converter (DAC) 220, and a ratio circuit 225. Tomeasure the amplitude of equalized signal Veq from equalizer 125,sampler 215 samples signal Veq with respect to a threshold voltage Vth,asserting a second sampled data signal Veq>Vth if the amplitude ofsignal Veq is greater than threshold voltage Vth at the sample instantdefined by sample clock Sclk. The amplitude of signal Veq can thus bemeasured by comparing the amplitude of signal Veq with a range ofthreshold voltages Vth. In this example, signal Veq is compared with arange of threshold voltages Vth to determine the highest thresholdvoltage Vth for which signal Veq exceeds voltage Vth (e.g., the highestvalue of threshold voltage Vth for which sampled data signal Veq>Vth isa logic one).

Ratio circuit 225 filters signal Veq>Vth by accumulating the number oftimes signal Veq>Vth is asserted for a desired number of samples. Inthis embodiment, a marker counter 235 establishes the selected number ofsamples, while a sample counter 230 accumulates the number of timessignal Veq>Vth is asserted. Sample counter 230 increments each time thesampled signal Veq is greater than the selected threshold voltage Vth,while marker counter 230 increments each time signal Veq is sampled.Marker counter 235 issues a carry signal Carry upon reaching the desirednumber of samples, at which time the contents of counter 230 isindicative of the number of samples for which signal Veq exceeded theselected threshold voltage Vth over the number of samples. The contentsof counter 230 divided by the count at which marker counter 235 issuescarry signal Carry is a measure of the probability that equalized signalVeq exceeded threshold voltage Vth at the sample instants. In oneembodiment, equalized signal Veq is considered to exceed thresholdvoltage Vth when the contents of counter 230 exceeds about 90% of thecount at which marker counter 235 issues the carry signal.

An AND gate 237 gates signal Veq>Vth using the enable signal from datafilter 150. Enable signal En is asserted to enable counters 230 and 235so that ratio circuit 225 only accumulates data in response to specifieddata patterns, as determined by data filter 150. When high frequencycomponents of Vin are attenuated relative to its low frequencycomponents, which could be expected to occur, for example, as Vintraveled from transmitter 105 to receiver 110 over channel 115, datafilter 150 may be configured to enable ratio circuit 225 in response toinput data patterns expressing relatively high frequencies (e.g., aseries of alternating ones and zeroes, as opposed to a series ofconsecutive ones or a series of consecutive zeroes). Data filter 150 canbe adjusted, in some embodiments, to enable ratio circuit 225, and thusamplitude detector 140, in response to different patterns, to measurethe equalized signal at different frequencies or to optimize thereceiver for different frequencies, for example.

In one embodiment, control logic 145 examines signals Carry and Sam foreach of a range of threshold voltages Vth to measure the amplitude ofsignal Veq for a given equalizer setting Eq. Control logic 145 thenrepeatedly measures the amplitude of signal Veq at different equalizersettings to find the equalizer setting that produces the highestamplitude of signal Veq. To accomplish this end, adaptive control logic145 includes a first register 240 that stores a digital threshold valueVth, a second register 245 that stores the value Vmax currentlyassociated with the highest value of signal Veq, a third register 250that stores the current equalizer setting Eq, and a fourth register 255that stores the equalizer setting Emax thus far producing the highestequalized signal amplitude. Though omitted for brevity, adaptive controllogic 145 may additionally convey control signals to ratio circuit 225that enable control logic to reset counters 230 and 235. In someembodiments, counters 230 and 235 can be programmed to sample differentnumbers of bits, 256, 128, 64, or 32 in one example.

FIG. 3 depicts a flowchart illustrating a convergence algorithm 300 thatmay be used by adaptive control logic 145 and amplitude detector 140, inone embodiment, to select an equalization setting for equalizer 125.FIG. 3 describes one method of operation of a receiver that may be usedas receiver 110 of FIGS. 1 and 2.

Convergence is initiated when an input signal is detected, at chipstart-up, for example (step 305), at which time registers 240, 245, 250,and 255 are each set to zero. Next, an amplitude-detect subroutine 307indirectly measures the amplitude of signal Veq by finding the highestthreshold voltage Vth for which the equalized input signal Veq isgreater than the threshold voltage Vth for e.g. about 90% of the sampledsymbols. To accomplish this in one embodiment, adaptive control logic145 first sets threshold count Vth to 1111, a value corresponding to thehighest threshold voltage Vth (step 310). Amplitude detector 140 thencompares signal Veq with threshold voltage Vth over 256 samples (step315), incrementing sample counter 230 each time signal Veq is found toexceed voltage Vth. If signal Veq does not exceed voltage Vth over 224times out of the 256 samples (decision 320), then count Vth isdecremented to reduce voltage Vth (step 325) and the comparison of step315 is repeated. This process is repeated until signal Veq exceedsvoltage Vth at least 224 times out of 256 samples (11100000 out of11111111), in which case threshold count Vth is held in register 240(step 330) to complete subroutine 307.

In the example of FIG. 2, marker counter 235 indicates a maximum countof 256 by asserting a carry signal Carry to adaptive control logic 145.The calculation of the sample ratio may be based upon other numbers ofsamples, and the ratio used to identify the signal amplitude of Veq maybe different. In some embodiments, the number of samples, the ratio, orboth are programmable. In one embodiment in which counters 230 and 235are each eight bits, the signal Sam from counter 230 is the AND of thehighest three bits, in which case Sam is a logic one when the value insampler counter 230 is at least 224 (binary 11100000). Thus, if both Samand Carry are logic one (Sa=1,1), then sampler counter 230 counted to atleast 224 by the time marker counter 235 reached a maximum count andthus generated a carry.

In the next decision 335, the current threshold count Vth is comparedwith count Vmax. If Vth is greater than Vmax, then the current equalizersetting is producing a higher equalized signal amplitude (e.g., a widereye) than the equalizer setting Emax, the equalizer setting previouslyassociated with the highest equalized signal amplitude. In that case,Vmax is updated with the value Vth and Emax is updated with Eq (step340). If Vth is not greater than Vmax, then the current equalizersetting is not producing a higher signal amplitude than whateverequalizer setting is currently associated with the highest signalamplitude. In that case, Vmax is held constant while the equalizersetting Eq is increased (step 345). Equalizer setting Eq is increased bytwo in this example, to more quickly span the range of equalizersettings employed during the convergence process. Other embodimentschange the equalizer settings in different steps, different orders, etc.

The next decision 350 determines whether the equalizer setting Eq iszero, indicating the count Eq has traversed the available range ofequalizer settings and rolled over to zero; if not, the process returnsto subroutine 307. This sequence of steps repeats over the range ofequalizer settings with step 340 accumulating counts Vmax and Emax,which respectively represent the highest value Vth for which signal Veqexceeds threshold voltage Vth for about 90% of sampled data and theequalization setting responsible for that maximum threshold setting.These final values of Vmax and Emax are held (step 355), completing theconvergence process.

Convergence algorithm 300 finds the optimal or a near-optimalequalization setting for a given communication channel, and may berepeated as needed to reacquire equalization settings. In someembodiments, for example, receivers adapted in accordance with someembodiments reacquire equalization settings each time power is applied.These and other embodiments may additionally benefit from adaptiveequalization schemes that continuously or periodically updateequalization settings to account for changes in the system operatingenvironment, such as in response to changes in temperature,supply-voltage, or other factors that impact receiver performance.

FIG. 4 is a flowchart illustrating a tracking algorithm 400 that may beimplemented by adaptive control logic 145 of FIGS. 1 and 2 in accordancewith one embodiment. Some embodiments periodically or continuouslyexecute a tracking algorithm after executing a convergence algorithm,such as, for example, the convergence algorithm 300 of FIG. 3, to adjustfor changes, such as noise, for example, in the signaling environment.Briefly, algorithm 400 measures the symbol amplitude of signal Veq forequalizer settings one count above and one count below the currentequalizer setting. If one of those settings produces a higher signalamplitude, the equalizer setting is adjusted to that improved setting.Other embodiments repeat the convergence algorithm to adapt toenvironmental changes or omit the convergence algorithm altogether,relying instead upon a tracking algorithm.

After tracking is initiated (step 405), control logic 145 begins bysetting register 250 to the value stored in register 255 (step 410). Theequalization setting for equalizer 125 is thus set to the value earlierdetermined to lead to the highest amplitude for signal Veq. If thecontents of register 250 is greater than zero (decision 415), thenregister 250 is decremented to reduce Eq by one (step 420). Amplitudedetect subroutine 307, described above in connection with FIG. 3, isthen called to measure the amplitude of signal Veq with the newequalizer setting. Per decision 425, if the new equalizer settingproduces a higher signal amplitude for Veq, as evinced by a thresholdvalue Vth greater than Vmax, then the contents of registers 245 and 255are updated with the respective contents of registers 240 and 250 (step430). The content of register 250 is then incremented (step 435),returning Eq to the value preceding the last instance of step 420.

If, at this time, the content of register 250 is less than the maximumcount (decision 440), then the content of register 250 is incrementedonce again (step 445). Amplitude detect subroutine 307 is once againcalled to measure the amplitude of signal Veq, this time to determinewhether a slightly higher equalizer setting provides a higher amplitudesignal Veq than the prior equalizer setting (decision 455). If so, thenthe contents of registers 245 and 255 are updated with the respectivecontents of registers 240 and 250 (step 460). The tracking algorithmthen returns to step 410. Tracking algorithm 400 can be turned offperiodically to save power.

FIG. 5 schematically depicts equalizer 125 of FIGS. 1 and 2 inaccordance with one embodiment. Equalizer 125 includes two nearlyidentical stages 500 and 505, the second of which is depicted as a blackbox for ease of illustration. Other embodiments include more or fewerstages. Equalizer stage 500 includes a pair of differential inputtransistors 515 and 520 with respective loads 525 and 530. Sourcedegeneration is provided by a resistor 535, a transistor 540, and a pairof capacitor-coupled transistors 545 and 550. The capacitance providedby transistors 545 and 550 is in parallel with resistor 535 andtransistor 540, so the net impedance between the sources of transistors515 and 520 decreases with frequency. As a consequence, the gain ofequalizer stage 500 increases with frequency. The resistance throughtransistor 540 can be adjusted to change the source-degenerationresistance, and thus to alter the extent to which the gain of equalizerstage 500 increases with frequency.

In an alternative embodiment, source degeneration is provided by one ormore floating metal-insulator-metal (MIM) capacitors connected inparallel with resistor 535. One such embodiment is detailed in theabove-referenced paper to Farjad-Rad et al. The MIM capacitors can beused instead of or in addition to capacitors 545 and 550.

A DAC 555 converts the digital equalization setting Eq from, in thisembodiment, adaptive control logic 145 to a gate voltage for transistor540. The value of the equalization setting thus determines theresistance between the drains of transistors 515 and 520, andconsequently the shape of the gain curve of equalizer stage 500. Ingeneral, the higher the resistance between the sources of transistors515 and 520, the more extreme the gain curve of stage 500 over thefrequency range of interest. In one embodiment, the output voltage fromDAC 555 decreases as setting Eq increases from 000000 to 100000,remaining constant for higher counts. These maximum counts representhighest resistance between the sources of transistors 515 and 520, andconsequently maximum equalization for stage 500. The output voltage froma similar DAC (not shown) in stage 505 remains high for counts up to100000, decreasing count-by-count for higher values. Thus, the lowestequalization setting (Eq=000000) represents the lowestsource-degeneration resistance for both stages 500 and 505, while thehighest equalization setting (Eq=111111) represents the highestresistance.

FIG. 6 schematically depicts a bias-voltage generator 600 for use withequalizer 125 of FIG. 5. A resistor 605 and transistors 610 and 615 forma half-circuit replica of equalizer stage 500, with the inputcommon-mode voltage Vin_com applied to the gate of transistor 610. Afeedback loop including an amplifier 620 and a pair of transistors 625and 630 sets the voltage on the inverting (−) terminal of amplifier 620equal to the voltage applied to the non-inverting (+) terminal. In anembodiment in which supply voltage Vdd is 1.2 volts, a resistor dividerprovides one-volt to the non-inverting terminal of amplifier 620. Theresulting bias voltage Vbias to stages 500 and 505 establishes aone-volt common-mode voltage for those stages. In some embodiments,lower common-mode voltages are avoided to ensure that transistors 515and 520 of FIG. 5 are always in saturation. The half circuit of FIG. 6can be scaled down, by a factor of eight in one example, to save power.

FIG. 7 schematically depicts DAC 220 and sampler 215 of FIG. 2 inaccordance with one embodiment. DAC 220 includes a sixteen-inputmultiplexer (MUX) 700 with four select terminals that receive a digitalrepresentation of the voltage threshold Vth from adaptive control logic145. The input terminals of MUX 700 connect to nodes of a voltagedivider network. A capacitor at each of the reference voltage stepsreduces the AC impedance of each node without using low resistances inthe ladder, which would result in high DC current consumption. A low ACimpedance causes the selected reference voltage to appear quickly onnode Vth for the next sampling period. The effective AC impedances ofthe input and reference lines are similar, as mismatches may affect thecomparison decision. In one embodiment, threshold voltage Vth can beadjusted over a range of from 0.8 volts to 1.2 volts. Threshold voltageVth is single ended in the embodiment of FIG. 7 to reduce the amount ofreference circuitry, though threshold voltage Vth may be differential inother embodiments.

In one embodiment, sampler 215 includes a pair of samplers 705 and 710,the outputs of which are combined by an OR gate 715 to produce outputsignal Veq>Vth. Both samplers 705 and 710 compare equalized signal Veqfrom equalizer 125 with the voltage difference between supply voltageVdd and threshold voltage Vth from DAC 220. These two referenceterminals are reversed between samplers 705 and 710 so that signalVeq>Vth is a logic one if the absolute value of Veq is greater than thedifference between voltages Vdd and Vth. Both samplers 705 and 710 aretimed to clock signal Sclk, which is in turn timed to the incoming data,so the comparison between the amplitude of voltage Veq and thedifference between voltages Vdd and Vth provides a measure of the eyeopening of the received data. Equalization settings are thus based uponmeasurements of the desired signal characteristic, in contrast to analogmethods that fail to distinguish noise from the valid signal.

FIG. 8 details an embodiment of clock reduction circuitry 200 of FIG. 2,which reduces the frequency of data clock Dclk by a factor of e.g. fourand creates sample clock Sclk edge aligned with data clock Dclk.Reducing the clock frequency simplifies the design of the amplitudedetector 140 and adaptive control logic 145, in some cases allowing themto be synthesized using a standard cell library. Edge aligner 210 alignsedges of sample clock Sclk with data clock Dclk so that amplitudemeasurements made by amplitude detector 140 are indicative of theamplitude detected by sampler 130 (FIG. 1).

An edge detector 800 compares the rising edges of data clock Dclk andsample clock Sclk, asserting a late signal Late if an edge of signalSclk occurs after a corresponding edge of signal Dclk and de-assertinglate signal Late if an edge of signal Sclk occurs before an edge ofsignal Dclk. A four-bit Up/Down counter 805 and a pair of AND gates 810and 815 collectively act as a digital low-pass filter. This filtergenerates a down signal DN to a second Up/Down counter 820 when the latesignal Late is asserted for eight more clock cycles than de-asserted,and generates an up signal UP when signal Late is de-asserted eight moreclock cycles than asserted. Counter 805 resets to a b=1000 state once itoverflows (b=1111) or underflows (b=0000).

The content of counter 820 controls the delay imposed by a phase picker825 to control the timing of sample clock Sclk relative to data clockDclk. Phase picker 825 includes a delay line 830 (e.g., a series ofbuffers) providing eight phases of clock signal Pclk to respective inputterminals of a multiplexer 835. Counter 820 is a saturating counter, sowhen reaching 111 (or 000) does not roll over to 000 (or 111), whengetting another up (or down) pulse. A multiplexer 835 selects one of theeight phases from tapped delay line 830, whose range spans at least halfa bit time (0.5 times one unit interval, or 0.5 UI, of data clock Dclk)across all corners of operation. In one embodiment, the granularity ofdelay line 830 does not increase more than 0.2 UI, leading to aquantization error of less than 0.1 UI. Trim bits to delay line 830 canbe included to cover a large range of the operating speeds. In oneembodiment, for example, the trim bits allow edge aligner 210 to coverthree regions of operation speeds: 4.25-6.25 Gbps, 2.125-3.125 Gbps, and1.062-1.56 Gbps.

FIG. 9 depicts data filter 150 of FIG. 1 in accordance with oneembodiment. Signal Veq is measured around signal transitions to bestmeasure the effects of equalization on signal-eye amplitude. Data filter150 enables amplitude detector 140 around transitions so that the outputof amplitude detector 140 accurately represents eye amplitude in thepresence of transitions. This configuration allows for optimization ofeye openings, or equalized-symbol amplitude, for minimum post-cursor (orpost-symbol) and pre-cursor inter-symbol interference (ISI).

Data filter 150 includes a pair of flip-flops 900 and 905 timed to dataclock Dclk to retain prior samples of a pair of incoming data bits d0and d1. Pattern detection circuitry 910 monitors the two prior datasamples from flip-flops 900 and 905 and the two most recent data samplesd0 and d1, producing a logic-one output signal in response to signaltransitions. A pair of flip-flops 915 and 920 provides a two-cyclepipeline delay to account for two previous bits and one bit after themonitored bit. A final flip-flop 925 captures the output of flip-flop920 on falling edges of sample clock Sclk and passes the resultingenable signal En to ratio circuit 225 (FIG. 2). Data filter 150 can beadapted to detect different patterns, and may be programmable in otherembodiments.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols are set forth to provide a thoroughunderstanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, the interconnection betweencircuit elements or circuit blocks may be shown or described asmulti-conductor or single conductor signal lines. Each of themulti-conductor signal lines may alternatively be single-conductorsignal lines, and each of the single-conductor signal lines mayalternatively be multi-conductor signal lines. Signals and signalingpaths shown or described as being single-ended may also be differential,and vice-versa. Similarly, signals described or depicted as havingactive-high or active-low logic levels may have opposite logic levels inalternative embodiments. As another example, circuits described ordepicted as including metal oxide semiconductor (MOS) transistors mayalternatively be implemented using bipolar technology or any othertechnology in which a signal-controlled current flow may be achieved.With respect to terminology, a signal is said to be “asserted” when thesignal is driven to a low or high logic state (or charged to a highlogic state or discharged to a low logic state) to indicate a particularcondition. Conversely, a signal is said to be “de-asserted” to indicatethat the signal is driven (or charged or discharged) to a state otherthan the asserted state (including a high or low logic state, or thefloating state that may occur when the signal driving circuit istransitioned to a high impedance condition, such as an open drain oropen collector condition). A signal driving circuit is said to “output”a signal to a signal receiving circuit when the signal driving circuitasserts (or de-asserts, if explicitly stated or indicated by context)the signal on a signal line coupled between the signal driving andsignal receiving circuits. A signal line is said to be “activated” whena signal is asserted on the signal line, and “deactivated” when thesignal is de-asserted. Whether a given signal is an active low or anactive high will be evident to those of skill in the art.

The output of the design process for an integrated circuit may include acomputer-readable medium, such as, for example, a magnetic tape, encodedwith data structures defining the circuitry can be physicallyinstantiated as in integrated circuit. These data structures arecommonly written in Caltech Intermediate Format (CIF) or GDSII, aproprietary binary format. Those of skill in the art of mask preparationcan develop such data structures from schematic diagrams of the typedetailed above.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example,

-   -   1. the amplitude of equalized signal Veq can be measured        indirectly by monitoring the output of a second equalizer with        input terminals coupled to terminals Vin_p and Vin_n and sharing        selected equalizer settings;    -   2. a single sampler could be used to recover data and measure        the amplitude of the equalized symbols (e.g., in a system that        supported operational and calibration modes);    -   3. embodiments of the invention may be adapted for use with        multi-pulse-amplitude-modulated (multi-PAM) signals; and    -   4. signals can be equalized to compensate for distortion other        than that caused by the low-pass nature of some channels (e.g.,        signals can be equalized to compensate for high-pass effect,        band-pass effects, or other types of distortion).    -   5. embodiments of the invention may measure the magnitude of        data symbols by detecting a current amplitude, voltage        amplitude, or both.        Moreover, some components are shown directly connected to one        another while others are shown connected via intermediate        components. In each instance the method of interconnection, or        “coupling,” establishes some desired electrical communication        between two or more circuit nodes, or terminals. Such coupling        may often be accomplished using a number of circuit        configurations, as will be understood by those of skill in the        art. Therefore, the spirit and scope of the appended claims        should not be limited to the foregoing description. Only those        claims specifically reciting “means for” or “step for” should be        construed in the manner required under the sixth paragraph of 35        U.S.C. § 112.

What is claimed is:
 1. A receiver comprising: an amplitude detector todetect an amplitude of an input signal of a varying input-signalvoltage, the amplitude detector comprising: a sampler to sample a firstnumber of voltage differences between the input signal and a thresholdvoltage and issue a decision signal indicative of when the input-signalvoltage exceeds the threshold voltage; and a ratio circuit coupled tothe sampler to accumulate a second number of times the decision signalindicates that the input-signal voltage exceeded the threshold voltageover the samples of the first number of voltage differences; and controllogic coupled to the amplitude detector, the control logic to adjust thethreshold voltage responsive to a ratio of the first number to thesecond number.
 2. The receiver of claim 1, further comprising anequalizer to receive and equalize a data signal, wherein the inputsignal comprises the equalized data signal.
 3. The receiver of claim 2,further comprising a data filter to issue an enable signal responsive toa pattern in the input signal.
 4. The receiver of claim 3, wherein theratio circuit accumulates the first number of voltage differencesresponsive to the enable signal from the data filter.
 5. The receiver ofclaim 3, wherein the pattern includes at least one transition of theinput signal.
 6. The receiver of claim 2, the control logic to adjustthe equalizer responsive to the second number of times the decisionsignal indicates that the input-signal voltage exceeded the thresholdvoltage over the sampled first number of voltage differences.
 7. Thereceiver of claim 1, wherein the sampler samples the voltage differencesin synchronization with a sample-clock signal.
 8. The receiver of claim7, further comprising a second sampler to sample the voltage differencesin synchronization with a data-clock signal.
 9. The receiver of claim 8,wherein the sample-clock signal is of a lower frequency than thedata-clock signal.
 10. The receiver of claim 9, further comprisingclock-reduction circuitry to derive the sample-clock signal from thedata-clock signal.
 11. The receiver of claim 1, wherein the input signalis differential.
 12. A method of measuring a voltage amplitude of aninput signal of a varying input-signal voltage, the method comprising:sampling a first number of voltage differences between the input-signalvoltage and a threshold voltage; issuing a decision signal indicative ofwhen the input-signal voltage exceeds the threshold voltage;accumulating a second number of times the decision signal indicated thatthe input-signal voltage exceeded the threshold voltage over the firstnumber of samples; and adjusting the threshold voltage responsive to aratio of the first number to the second number.
 13. The method of claim12, further comprising receiving and equalizing a data signal to producethe input signal.
 14. The method of claim 13, further comprisingdetecting a pattern of the input signal and issuing an enable signalresponsive to the pattern.
 15. The method of claim 14, furthercomprising accumulating the samples of the first number of voltagedifferences responsive to the enable signal.
 16. The method of claim 14,wherein the pattern includes at least one transition of the inputsignal.
 17. The method of claim 12, the sampling of the input signalsynchronized with a sample-clock signal.
 18. The method of claim 17,further comprising sampling the voltage differences in synchronizationwith a data-clock signal.
 19. The method of claim 18, wherein thesample-clock signal is of a lower frequency than the data-clock signal.20. The method of claim 19, further comprising deriving the sample-clocksignal from the data-clock signal.